1. Field of the Invention
This invention relates to cardiac pacing systems and, more particularly, to dual chamber pacing systems designed to optimize operation in a synchronous tracking mode.
2. Description of the Prior Art
The advantages of dual chamber pacing, and more particularly pacing in different modes which are selected in response to different patient conditions, is now well recognized in the art. Early pacing systems were solely ventricular, and were sufficient for management of patients with complete heart block and Stokes-Adams attacks. However, ventricular demand pacemakers are not equipped to take advantage of atrial activity, and thus are limited in their efficiency. Subsequently, atrial synchronous, ventricular pacemakers were introduced, having a lead for sensing P signals from the atrium and another for pacing the ventricle after a suitable P-R (A-V) interval. Such a pacemaker, e.g. VDI or VDD, allows the atrium to control the heart's response rate, the ventricle being paced at the atrial rate up to a predetermined upper rate limit. Such synchronous pacers have incorporated means for dealing with high atrial rates, including "block" and "Wenckebach" techniques.
Another form of A-V or dual chamber pacer that has been utilized is the sequential pacemaker (DVI), which paces both the atrium and the ventricle with an appropriate A-V delay which is timed by the pacemaker. A number of commercial pacemakers have been introduced which are programmable to these and other known pacing modes. Each of the various operating modes is particularly adapted to certain circumstances that may arise in a given patient.
Since the dual sense-dual pace DDD pacemaker became commercially available, it has gained favor for the reason that it compensates for many of the disadvantages of other pacemaker modes. The classic DDD pacemaker is described in U.S. Pat. No. 4,920,965, Funke et al., in some detail. See also U.S. Pat. Nos. 4,539,991 and 4,554,921, incorporated herein by reference, which disclose other forms of DDD-type pacemakers.
More recently, the DDDR pacemaker has come to prominence. In this type of pacemaker, there is provided one or more sensors which enable the pacemaker to be rate responsive, such that the pacing interval, or escape interval, is varied as a function of one or more sensed rate-indicating parameters, rather than being fixed at a programmed value. In the DDDR pacemaker, both atrial and ventricular natural beats may occur so long as they occur prior to the respective rate responsive escape interval. See U.S. Pat. Nos. 4,467,807 and 4,951,667, which are illustrative of dual chamber rate responsive pacemakers.
There have also been disclosed multi-mode pacemaker designs having means for switching modes in response to changing patient conditions. Most dual chamber pacemakers are programmable to distinct modes, or switch automatically from one mode to another under certain prescribed conditions. See, for example, U.S. Pat. No. 4,527,568, and U.S. Pat. No. 4,920,965. However, there remains a substantial need in the pacing art for sensing the conditions under which a dual chamber pacemaker can or should be controlled to change mode, and for providing optimum flexibility for blending two or more modes of operation. Thus, instead of forcing the pacer to operate in a distinct mode until patient history enables switching to another distinct mode, the pacer would optimally be enabled to react on a cycle-to-cycle basis to sensed events. For example, while it is desirable to synchronize a delivered ventricular stimulus to a sensed atrial signal whenever possible, at the same time the pacemaker should be controlled to adopt another more optimum response whenever desired. Thus, if an atrial sensed beat occurs at too high a rate, or if retrograde conduction is determined, the pacemaker should have maximum flexibility for responding to this situation.
Reference is made to U.S. Pat. No. 5,247,930, assigned to the assignee of this invention, and incorporated herein by reference. This patent discloses a dual chamber pacing system which provides improved physiological synchronous tracking of atrial beats, and a means for continuously determining a physiological rate. The physiological rate normally tracks sensed atrial signals, but does not follow atrial signals found to be non-physiological. Thus, as long as the atrial signals are physiological, the "phys rate" is substantially equal to the sensed atrial rate. The referenced system establishes dynamic decision rates which are coupled to the physiological rate, including a dynamic tracking limit (DTL) and a dynamic pacing limit (DPL), which two limits define a physiological range. The dynamic range permits the pacemaker to adapt operation to varying atrial rates, and continue tracking a wider overall range of physiological atrial signals.
In the system of the referenced patent, the dynamic pacing limit is a son of flywheel rate, and represents the rate at which pace pulses are delivered in the absence of physiological atrial rates that can be synchronized. For example, the dynamic pacing limit may be maintained at 15 ppm below the phys rate as long as physiological atrial signals are sensed. However, if a real tachycardia situation occurs, the pacer responds with asynchronous pacing, i.e., the ventricle is paced without relation to the atrial senses. When this happens, it is necessary to decrement the flywheel rate, or DPL. In the referenced system, this is done by decrementing phys rate (along with DPL) until both reach the lower rate limit (LRL), or the sensor rate, whichever is higher.
However, a problem occurs in that not all disturbances are of a nature that the phys rate and the decision rates should be decremented down toward LRL. In particular, it has been found that it is desirable not to change the phys rate, and consequently the DTL, upon short disturbances. Thus, in event that there has been a stable normal sinus rate within the physiological band, and then there occurs a short disturbance such as a few PAC's or several undersensed beats in a VDD, it is desirable not to drop the entire physiological band. If the phys rate is decremented immediately toward LRL, carrying DTL with it, then when the stable normal sinus rate reappears, it might be outside (above) the physiological band, such that the pacemaker could not track it. There thus remains a substantial need in dual chamber pacemakers which dynamically adapt to a patient's physiological rate, to optimally react to short term disturbances of the sensed atrial rate, enabling the pacemaker to more quickly provide a synchronous response to a reappearing physiological sinus rate.